PC pumps and mud motors (“Moineau pumps”) of conventional design have a moulded elastomeric insert bonded firmly to the inside of a cylindrical case, usually made of steel. This comprises the stator of the pump or motor unit. The inside shape of the elastomer is formed with a cavity that has a helical characteristic that mates with a helically-shaped stator. Interference between the two components creates seal lines that contain cavities of fluid which progress in the axial direction when the rotor is rotated relative to the stator. If rotational power is applied to the rotor, the assembly functions as a pump against differential pressure. If differential pressure is applied across the assembly, rotary power is extracted from the rotor and the assembly functions as a motor.
When formed inside of a cylindrical case out of elastomer, the shape of the stator cavity requires the elastomer thickness to vary around the circumference. The locations where the thickness is greatest are subjected to the largest distortional elastomer stresses during operation.
Cyclic stress developed in the elastomer by the seal location moving back and forth, or around the stator cavity generates heat in the core of the elastomer, which must be removed by conduction through the elastomer, either to the outer stator casing or to the inner surface of the elastomer where it is convected to the transported fluid. In conventional designs, the largest heat-generation rate occurs where the ability to remove the heat is lowest. If it over-heats, the elastomer can fail and the function of the pump/motor is compromised. This has been a significant limitation in the performance and design of progressing cavity pumps and motors, and has led to the development of “uniform-thickness” elastomer designs, where the internal casing profile is provided to closely match the required stator cavity profile, and a relatively thin layer of elastomer is moulded to this surface to provide the final stator cavity geometry.
This approach has several advantages, including reduced heat generation and swelling characteristics. The primary disadvantage is the cost of providing the relatively complicated internal profile from the high-strength material of the casing. Several approaches have been developed, including cold-rolling techniques, machining of the internal profile, and the use of extrusion techniques to produce the required geometry. These approaches are expensive, particularly in the lengths required for PC pump/motor applications. Some of these techniques are described in Canadian Patents 2,315,043 (Krueger et al), 2,333,948 (Underwood et al) and U.S. Pat. No. 6,427,787.
Furthermore, while these patents identify certain advantages to be gained from thin walled stators, the methods of manufacture described, are not amenable to close tolerance control for such stators.